There are various types of motors such as a brushed DC motor which uses a magnet, a brushless DC motor, an induction motor which does not use a magnet, and a switched reluctance motor. The switched reluctance motor is hereinafter referred to as an SR motor. Since the SR motor does not use a magnet, it has the advantages of (1) being at structurally simple, low coat, and robust, (2) being capable of high-speed rotation, (3) being usable at high temperatures, and (4) being easy to recycle.
On the other hand, the SR motor has the disadvantages of (1) having large torque ripples due to the salient pole structure and having large drive noise, (2) having poor power factor, (3) requiring stringent precision in processing metal plates for forming gaps, etc., (4) requiring a dedicated drive circuit, and (5) being difficult to control due to strong non-linearity. In order to drive the SR motor, it is required to switch the energization phase based on rotational angle. It is possible to control the resulting torque by controlling the energization timing, current waveforms, etc.
For example, Japanese patent application publication H09-182490 suggests the following solution for reducing drive noise which is one of the disadvantages of the SR motor. That is, in a configuration in which energization of the motor winding is permitted based on a turn-on of a signal determining the timing for exciting the motor winding and in which the current value flowing through the motor winding is controlled based on a PWM signal, the timing for starting the PWM signal is synchronized with the risking of the signal determining the excitation timing from off to on.
However, it is not be possible for the above described approach to control current in regions where the motor rotates at high speeds. In case of a three-phase SR motor having 12 stator salient poles and 8 rotor salient poles for example, the energization phase needs to be switched 24 times per 1 rotation. The time required to switch the energization phase amounts to 416.7 μs at 6000 rpm and 41.7 μs at 60000 rpm. In such case, 8 pulses can be outputted at 6000 rpm but only 1 pulse can be outputted at 60000 rpm when the frequency is 20 kHz.